Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI signals for reconstructing an image of an object are obtained by placing the object in a magnetic field, applying magnetic gradients for slice selection, applying a magnetic excitation pulse to tilt nuclei spins in the desired slice or volume, and then detecting MRI signals emitted from the tilted nuclei spins while applying readout gradients. The detected signals may be envisioned as traversing lines in a Fourier transformed space (k-space) with the lines aligned and spaced parallel in Cartesian trajectories or emanating from the origin of k-space in spiral trajectories.
An MRI may be used for scanning a patient's brain. The MRI may be useful for measuring development of the brain, particularly for scanning white-matter within the brain. White matter is a component of the central nervous system and consists of myelinated axons. Myelin acts as an insulator around nerve cells and increases the speed of transmission of nerve signals. The multilayered myelin sheath wrapping around nerve axons is essential for proper functioning of the central nervous system. Abnormal myelination leads to a wide range of neurological diseases and developmental disorders. MRI is the preferred reference test for diagnosing and monitoring the evolution of white-matter development and related diseases due to its excellent soft tissue contrast, high spatial resolution, and non-radioactive nature. However, conventional MRI methods may not provide satisfactory accuracy for quantifying the myelinated axons, and determining, for example, the amount of myelinated axons present in a subject's brain.